Internal reflux computer for fractionation control



Jan. 23, 1962 w. MORGAN INTERNAL REFLUX COMPUTER FOR FRACTIONATION CONTROL Filed March 4, 1959 2 Sheets-Sheet l 9 n 2 C R will! F m R as ii w fi D R A O 4 5 T 2 2 M u u n U (J M m R w c m C WU L A m| l I l l I l I IIOD III F mm m A R w 2 T. 2 6 u 2! 8 R m {l m 4 T U E U r 1% w 5 M 1 I l l l I l l l|L 3 z F FEED IN V EN TOR.

L.W. MORGAN FLOW TRANSDUCER HMJLM+ AAAA SERVO MOTOR 49 FRC A TTORNEYS Jan. 23, 1962 w. MORGAN 3,018,229

INTERNAL REFLUX COMPUTER FOR FRACTIONATION CONTROL Filed March 4, 1959 2 Sheets-Sheet 2 United States Patent 9 3,018,229 INTERNAL REFLUX COMPUTER FOR FRACTIONATION CONTROL Lyman W. Morgan, Golden, Colo., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Mar. 4, 1959, Ser. No. 797,240 8 Claims. (Cl. 202-40) This invention relates to the computation of internal reflux in a fractionation column. In another aspect it relates to control systems for fractionation columns which are based on computations of internal reflux.

In recent years an increasing use has been made of fan coolers for condensing overhead vapors from fractionation columns. However, this type of cooler has resulted in a rather serious operating problem because it is diflicult to control the exact amount of cooling provided. Such schemes as fan speed control, variable pitch fan blade control and hot vapor by-pass control have been employed in an attempt to solve this problem, but have not been entirely satisfactory. Sudden atmospheric temperature changes, such as occur during a rainstorm, for example, result in condensation of vapor within the column because of sub-cooling of the reflux below its bubble point. This results in an increase in overhead product purity at the expense of a decreased overhead product rate and, therefore, a decrease in column throughput.

In accordance with the present invention there is provided a novel system for computing the amount of internal reflux in a fractionation column. Internal reflux constitutes the external reflux returned to the column plus the vapor which is condensed near the top of the fractionation column by the sub-cooled external reflux. This computation is made from a measurement of the rate of flow of the external reflux and a measurement of the temperature differential between the external reflux returned to the column and a region near the top of the column. Signals representative of these two measurements are combined to provide a measurement of the internal reflux in the column. A signal representative of this internal reflux can be employed to control the operation of the column to maintain desired steady state operating conditions. This control can be made automatically or by an operator from the information provided.

Accordingly, it is an object of this invention to provide a system for computing the internal reflux in a fractionation column.

Another object is to provide a control system for a fractionation column which is based upon a measurement of the internal reflux in the column.

Other objects, advantages and features of the invention should become apparent from the following detailed description which is taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a schematic representation of a fractionation column having the computer and control system of this invention incorporated therein.

FIGURE 2 is a modified form of the differential temperature measuring means employed in the control system of FIGURE 1.

FIGURE 3 is an electrical embodiment of the computer and control system of this invention.

FIGURES 4a and 4b are graphical representations of the operation of a column Without and with the control system of this invention, respectively.

Referring now to the drawing in detail and to FIGURE 1 in particular, there is shown a conventional fractionation column which is provided with a number of vaporliquid contacting trays. A fluid mixture to be separated is introduced into column 10 through a conduit 11 at a predetermined rate which is maintained by a flow con- "ice troller 12 that adjusts a valve 13. Steam, or other heating medium, is circulated through the lower region of column it through a conduit 14 at a predetermined rate which is maintained by a flow controller 15 that adjusts a valve 16. A kettle product stream is withdrawn from the bottom of column 10 through a conduit 17. The rate of product withdrawal through conduit 17 is regulated in response to a liquid level controller 18 which adjusts valve 19. Vapors are withdrawn from the top of column lil through a conduit 21 which communicates with an accumulator 22 through a condenser 23. The present invention is particularly useful when this condenser is a fan cooler, but the invention is by no means restricted to such a condenser. Condensed liquid in accumulator 22 is removed through a conduit 24 which communicates with the inlet of a pump 25. Pump 25 returns a portion of this liquid to the upper region of column 10 through a conduit 26. The flow of liquid through conduit 26 is referred to hereinafter as the external reflux. The remainder of the liquid removed from accumulator 22 constitutes the overhead product and is directed through a conduit 27. The flow through conduit 27 is regulated by a liquid level controller 28 which adjusts a valve 29 in response to the liquid level in accumulator 22.

In order to explain the operation of the internal reflux computer and control system of this invention, an equation which is representative of the internal reflux in a fractionation column will be derived.

The material balance at the top tray of the fractionato-r can be expressed:

where R =mass flow of liquid entering top tray (external reflux) V =rnass flow of vapor entering top tray R =mass flow of liquid leaving top tray (internal reflux) V =mass flow of vapor leaving top tray.

The heat balance at the top tray can be expressed:

e e+ i i i+ o where h =enthalpy of external reflux h =enthalpy of internal reflux H=enthalpy of vapor streams (assumed to be equal) where C =specific heat of the external reflux stream AT=the difference in temperature at the top tray and external reflux Equation 3 can be substituted into Equation 2 to eliminate H and rewritten:

Equation 4 can be substituted into Equation 5 to eliminate h and rewritten:

( i+ 1 0) 1( 1 e)+ e p From Equation 1 it is known:

i' o i e Equation 7 can be substituted in Equation 6 and rewritten to obtain:

R.= R,+ B g n T (s) The computer of this invention solves Equation 8. The term R is measured by a flow transducer 35 which establishes a signal representative of the rate of flow through conduit 26. The term AT is measured by comparing the temperature of the reflux in conduit 26 with the temperature at or near the top of column 10. In FIGURE 1, this measurement is made by a differential temperature transducer 36. Transducer 36 compares the temperature of the reflux in conduit 26 with the temperature in the column a few trays below the reflux conduit. This location is advantageous in some operations because the temperature a few trays down from the reflux entry is stabilized to a certain extent. However, the temperature in conduit 26 can also advantageously be compared with the temperature in conduit 21, as illustrated in FIG- URE 2.

The output signal from transducer 36 is multiplied by a constant representative of the term The output signal from flow transducer 35 is-multiplied by the output signal from multiplier 37 by a second multiplier 34 to establish the term B TA T The output signal from multiplier 34 is added to the output signal flow transducer 35 by an adder 39 to establish a signal representative of the internal reflux This signal is applied to the set point of a flow recordercontroller 40 which regulates a valve 41 in reflux conduit 26. Valve 41 is regulated'in a manner so as to maintain the computed internal reflux constant at a predetermined value. 'If the computed internal reflux should increase, valve 41 is set to reduce the flow through conduit 26; if the computed internal reflux should decrease, valve 41 is set to increase the flow through conduit 26.

An electrical embodiment of the computer of FIG- URE l is illustrated in FIGURE 3. The two temperature measurements are made by respective thermocouples 45 and 46 which have their outputs connected in electrical opposition to the input of an amplifier 47. The output signal of amplifier 47, which advantageously is employed to increase the output signal from the thermocouples to a convenient level, is applied across a potentiometer 48. The contactor and one end terminal of potentiometer 48 are connected to the respective input terminals of a servo motor 49. The contactor of potentiometer 48 is set so that the voltageapplied across the end terminals thereof, whch is representative of AT is multiplied by the term Servo motor 49, which can advantageously be of the type described in Electronic Control Handbook, Batcher and Moulic, Caldwell-Clements, Inc., New York, 1946, page 298, adjusts the contactor of a multiplying potentiometer 50. The output signal of flow transducer 35, which can be any conventional flow measuring instrument that provides an electrical output signal representative of flow, is applied across the end terminals of a potentiometer 50. The voltage at the contactor of potentiometer 50, taken with respect to the grounded end terminal, is thus representative of the term This voltage is applied through an input resistor 51 to the first terminal of a summing amplifier 52. The output signal from transducer 35, representative of the term R is also applied to amplifier 52 through an input resistor 53. Amplifier 52 is provided with a feedback resistor 54. The output signal of amplifier 52, which is thus representative of the internal reflux R is applied to controller 40.

Other types of apparatus can be employed for the various elements of the computer of FIGURE 1. For example, transducers 35 and 36 can establish output pneumatic pressures which are representative of the flow and temperature differential, respectively. Multipliers 37 and 34 can be force bridges. of the type disclosed in US. Patent 2,643,055, for example. Adder 39 can be a conventional pneumatic summing relay. Thus, both electrical and pneumatic elements can be employed to advantage in the computer of this invention.

It has been found that this invention provides substantially smoother operation in a fractionation column. FIGURE 4a is a. graphical representation of the kettle product flow rate of a column employed to separate a feed mixture consisting essentially of normal pentane and isopentane. FIGURE 4b illustrates the flow rate of the same stream when the controller of this invention was added to the column. It can be seen that fluctuations in flow are substantially reduced by the controller of this invention.

In view of the foregoing description, it should be evident that there is provided in accordance with this invention a novel computer for measuring the internal reflux in a fractionation column. It has been found that control systems utilizing this computer to regulate the rate of external reflux have stabilized the operation of fractionation columns to a much larger degree than was possible with previously known control systems. The computer of this invention utilizes a minimum number of conventional components and can readily be adapted for use in existing columns without extensive modifications.

While the invention has been described in conjunction with present preferred embodiments, it should be evident that it is not limited thereto.

What is claimed is:

1. In a fractionation system wherein a feed mixture of two or more components is subjected to a medium which establishes two phases of the feed mixture which are passed in countercurrent relationship to onev another in a contacting means and wherein at least a part of constituents removed from the contacting means is returned thereto as external reflux, a control system comprising means to establish a first signal representative of the rate of flow of external reflux to said contacting means, means to establish a second signal representative of the temperature difference between said constituents removed from said contacting means and said external reflux returned to said contacting means, means responsive to said first and second signals to establish a third signal representative of internal reflux in said contacting means, and means responsive to said third signal to decrease the flow of said external reflux when the measured internal reflux increases and to increase the flow of said external reflux when the measured internal reflux decreases, thereby to control the operation of said fractionation system to maintain a predetermined separation between components of said feed mixture.

2. The control system of claim 1 wherein said means to establish said third signal comprises means to multiply said second signal by a constant term to establish a fourth signal, means to multiply said first signal by said fourth signal to establish a fifth signal, and means to sum said first and fifth signals to establish said third signal.

3. In a fractionation system wherein a feed mixture of two or more components is directed to a fractionation column, a vapor stream is removed from the top of said column, said vapor stream is cooled to condense at least a part of same, and at least a part of the resulting condensate is returned to the column as external reflux, a control system comprising means to establish a first signal R representative of the rate of flow of said external reflux to said column, means to establish a second signal AT representative of the temperature difference between said vapor stream and said external reflux, means responsive to said first and second signals to establish a third signal R representative of internal reflux in said column, said third signal being established from the relationship:

where C is the specific heat of said external reflux and A is the heat of vaporization of liquid in the top of said column, and means responsive to said third signal to decrease the flow of said external reflux when the measured internal reflux increases and to increase the flow of said external reflux when the measured internal reflux decreases, thereby to control the operation of said column.

4. In a fractionation system wherein a feed mixture of two or more components is directed to a fractionation column, a vapor stream is removed from said column, said vapor stream is cooled to condense at least a part of same, and at least a part of the resulting condensate is returned to the column as external reflux through a conduit means having a control means therein to regulate flow of such external reflux, apparatus to compute the internal reflux in said column comprising means to establish a first signal R representative of the rate of flow of said external reflux to said column, means to establish a second signal AT representative of the temperature difference between said vapor stream and said external reflux, and means responsive to said first and second signals to establish a third signal R representative of internal reflux in said column, said third signal being established from the relationship:

where C is a constant representative of the specific heat of said external reflux and A is a constant representative of the heat of vaporization of liquid in the top of said column.

5. The apparatus of claim 4 wherein said apparatus to compute comprises means to multiply said second signal by a constant representative of to establish a fourth signal, means to multiply said fourth signal by said first signal to establish a fifth signal, and means to add said first and fifth signals to establish said third signal.

6. The apparatus of claim 4 wherein said means to establish said second signal comprises first temperature sensing means disposed in said conduit means adjacent said column, second temperature sensing means disposed in said vapor stream adjacent said column, and means to establish a signal representative of the temperature difference between said second and first temperature sensing means.

7. The apparatus of claim 4 wherein said means to establish said second signal comprises first temperature sensing means disposed in said conduit means adjacent said column, second temperature sensing means disposed in said column adjacent the top thereof, and means to establish a signal representative of the temperature difference between said second and first temperature sensing means.

8. In a fractionation process wherein a feed mixture of two or more components is directed to a fractionation zone, a vapor stream is removed from the top of said zone, said vapor stream is cooled to condense at least a part of same, and at least a part of the resulting condensate is returned to said fractionation zone as external reflux; a control method which comprises the steps of measuring the rate of flow of said external reflux and establishing a first signal R representative thereof, measuring the temperature diflerence between said 'vapor stream and said external reflux and establishing a second signal AT representative thereof, combining said first and second signals so as to establish a third signal R; which is equal to the quantity:

R,+ B g n T References Cited in the file of this patent UNITED STATES PATENTS Noel Nov. 12, 1929 Boyd Jan. 1, 1952 OTHER REFERENCES Instruments and Process Control, published by N.Y. State Vocational and Practical Arts Association, 1945 (pages -185).

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3 018329 January 23 1962 Lyman W. Morgan ied that error appears in the above numbered pat- It is hereby certif t the said Letters Patent should read as ent requiring correction and the corrected below.

Column 5 lines 20 and 21 after "15" each occurrence lnsert a constant representative of same column 'line 29 after "from" insert the top of Signed and sealed this 12th day Of June 1962.

( SEAL) Attest:

. DAVID L. LADD ERNEST W. SWIDER Commissioner of Patents Attesting Officer 

8. IN A FRACTIONATION PROCESS WHEREIN A FEED MIXTURE OF TWO OR MORE COMPONENTS IS DIRECTED TO A FRACTIONATION ZONE, A VAPOR STREAM IS REMOVED FROM THE TOP OF SAID ZONE, SAID VAPOR STREAM IS COOLED TO CONDENSE AT LEAST A PART OF SAME, AND AT LEAST A PART OF THE RESULTING CONDENSATE IS RETURNED TO SAID FRACTIONATION ZONE AS EXTERNAL REFLUX; A CONTROL METHOD WHICH COMPRISES THE STEPS OF MEASURING THE RATE OF FLOW OF SAID EXTERNAL REFLUX AND ESTABLISHING A FIRST SIGNAL RE REPRESENTATIVE THEREOF, MEASURING THE TEMPERATURE DIFFERENCE BETWEEN SAID VAPOR STREAM AND SAID EXTERNAL REFLUX AND ESTABLISHING A SECOND SIGNAL $T REPRESENTATIVE THEREOF, COMBINING SAID FIRST AND SECOND SIGNALS SO AS TO ESTABLISH A THIRD SIGNAL R1 WHICH IS EQUAL TO THE QUANTITY: 